Local files super laggy

brado If you still have k-lite installed.

IN CASE this ffs up your other video players, you can restore your backup you make at the start, point 1. Actually, might even consider making a windows restore point for good measure. That way you can also undo point 3. The codec tool backup might not have that saved.

go to start, type codec tweak tool

In Media Foundation nothing should be checked after reset. Double check.
Restart all VR/video/browser software you might have open. Test if playback in SLR/Deo is fixed.

If still not fixed, open codec tool again, in lower left go to Preferred decoders.
switch all h.264 h.265 mp4/mpg4 to LAV, both 32 & 64 bit.
Audio you can leave; Unless it turns out it's an audio codec issue. But first try without changing audio settings. All SLR videos use AAC. Pretty much everything media supports it.

Rakly3
Since you do have it run smoothly on WMR there must be a workable combination with all 3 outputs.
Your headset can do up to 90Hz, but your 1070 isn't gonna do 90Hz over 4K (I think, don't have the EXACT numbers for each dimention on front of me). But at 7K-8K your max is 60Hz on 1 display.

If you have a CPU with built in graphics you could put one display on that one. Your 1070 can still render graphics on that display even if plugged in other "card". Just make sure the 1070 has the Primary display. (Like litterally in your windows display settings)

brado that's sadly not how it works. Also has nothing to do with power consumption.
Unless you're using viewport, you are rendering the full 7K no matter how small your display is.

Wait, is your WMR using viewport?
90Hz over 4K+, plus your other displays is not gonna work.

Even if a video is higher resolution the displays on the headset will still have 14402880 pixels to render. I cannot find a setting in SteamVR to lower the refresh rate.

That's when you render graphics on the GPU. Here we are decoding image frames on the hardware codec chip.

brado I'm 90% sure it's refresh-rate related.
Sound is likely also over HDMI.
Just lower the refresh rate of SteamVR.

Power should not be an issue, assuming your monitors have their own power supply. But as you said, it works WMR.
The interface of DeoVR (Unity) also has to be rendered aside from your monitors and VR video.

You could try to force the interface to run on your IGFX, I'm not sure that will fix it though, or could even create other issues. But it's always worth a try, you can undo it whenever you want.
Like I run my browser on my IGFX (which display doesn't matter. Doesn't even need a display attached.)

Settings > Display > Graphics Settings > Desktop app > Browse > \Steam\steamapps\common\DeoVR Video Player\DeoVR.exe > Pick prefered GPU.

This shouldn't change which GPU decoder is used! That's set in the codec settings. Only the rendering of the app is changed.

This can also help in game performance if you change your background apps from GPU.

Another solution could be using QuickSync decoder for the video. Then you are running the video on the IGFX. Though the Nvidea decoder is 'better'.

brado
Glad to hear it gotten a bit better though.
Have you tried unplugging your 2nd monitor, just for testing?

brado Maybe it gives only 30 hz per eye then or something?

To my knowledge no, as far your PC is concerned it's one display. But I never actually thought of checking that, something for my to-do list! Could be different depending on HMD. It would however still be 2x 60Hz though.

brado PS, I also uninstalled HEVC but it didn't make any difference

If you are using AVC (h264) videos it indeed doesn't make a difference.

Etshy if the videos are h265/HEVC then you must have the HEVC codec installed, correct. You can whatever you want with the k-lite pack, but without that codec it's never gonna work.

h264/AVC does work without purchasing the HEVC codec.

Etshy Did you have the free version installed prior, but it still wouldn't work? Just curious if MS disabled the free version from working completely.